In Direct Sequence Code Division Multiple Access (DS-CDMA) cellular telecommunication systems, a cellular base station communicates with multiple mobile units while maintaining exclusive communication links with the mobile units. Each exclusive link is assigned a code that is orthogonal with all other codes. The most commonly used type of code is the Walsh code. The Walsh code has a property that all its member codes are orthogonal with each other. This property of Walsh codes allows a base station in DS-CDMA system to transmit a composite of carrier signals, where each carrier signal is coded with an assigned Walsh code. Each receiving unit extracts desired information based on the assigned Walsh code and ignores other orthogonal Walsh codes.
The composite carrier signal is amplified through a linear power amplifier that has a finite peak power capacity; however, the finite peak power of the composite carrier signal may be substantially large. In such a system, the linear power amplifier should have an arduous linear operating region, that in turn places extreme burden on designing and maintaining the linear power amplifier within its linear operating requirements. As a result, the cost of the linear power amplifiers, in DS-CDMA system, are substantially increased.
A clipping technique is a commonly used method for relaxing linear power amplifiers operating requirements. In this method, if the composite carrier signal peak amplitude is above a certain level, the signal amplitude is limited prior to being input to the transmitter linear power amplifier.
For example, with reference to FIG. 1, a DS-CDMA transmitter system for transmitting a composite carrier signal including a clipper 103 is shown. A plurality of channels 101-1 through 101-L are summed together in a summer 105 to produce a composite signal 102 x(n) where the plurality of channels 101-1 through 101-L are respectively coded with a plurality of assigned Walsh codes 1 through L. Normally in a given Walsh code space, the total number of available Walsh codes are more than what actually are assigned at a given time.
The composite signal 102 x(n) is input to the clipper 103. The clipper 103 clips the peaks of the composite signal 102 x(n), and produces a clipped composite signal 104 y(n). However, the clipping technique adds a significant signal degradation distortion, as commonly known as remodulation effect, at the assigned Walsh codes to the composite signal 104 y(n).
Therefore, a need exists for a solution that allows reducing peak-to-average power ratio of a composite carrier signal without having the detrimental effects of the clipping technique.